I should think a dozen, assuming water is not too hard, or too warm, and depending on other fishes being kept.
Most of the corydoras appreciate softer rather than harder water, and temp's not too much above 76 degree's F.

It truly depends on the species that you put in it. They have a range from a 1" Corydoras pygmaeus to 4" Scleromystax barbatus. I will assume you are considering the average of 2" per adult fish. I would say around the 15 to 18 range.

I keep 9 in 30" long tank and its a good fit. 5 Corydoras sterbai at ~3" and 4 Corydoras similis at 2"

Since we are talking about Corys in common name terms... anything in the subfamily Corydoradinae is a Cory. :)

Hey Byron, On the Tropical Fish Profile for Corydoras similis, can you label the third picture from the left as Corydoras ourastigman. Tried to edit it but didnt see the option to edit pictures. I noticed the pic was not a similis and was going to message you to remove it but noticed that you referenced it in the article. Also, I tried to message you directly but was unable to send the message... some bb code error.

Hey Byron, On the Tropical Fish Profile for Corydoras similis, can you label the third picture from the left as Corydoras ourastigman. Tried to edit it but didnt see the option to edit pictures. I noticed the pic was not a similis and was going to message you to remove it but noticed that you referenced it in the article. Also, I tried to message you directly but was unable to send the message... some bb code error.

I agree the photo is not C. similis, and it is C. ourastigma [without an "n"], but in the first paragraph under Description is does say this. I included the photo to illustrate the close similarity of species.